COFF: Correctly handle relocations against early discarded sections.

Don't crash if we encounter a reference to an early discarded section
(such as .drectve). Instead, handle them the same way as sections
discarded by comdat merging, i.e. either print an error message or
(for debug sections) silently ignore the relocation.

Differential Revision: https://reviews.llvm.org/D40235

llvm-svn: 318689